a. Field of the Invention
The present invention relates to electronic calibration equipment for verifying the high frequency characteristics of electronic test equipment, including oscilloscopes and time interval analyzers.
b. Related Art
In order to verify the high frequency characteristics of electronic equipment such as oscilloscopes, it is necessary to use a test signal generator that provides square edge test signals that have edges with a rise time and/or fall time at least as fast as the oscilloscope""s specified rise time and/or fall time. The test signal should have aberrations from an ideal square edge significantly better than the specification of the equipment being tested. A complication arises with test equipment such as a conventional oscilloscope (rather than a sampling oscilloscope) that has a wide dynamic range at its input. Conventional oscilloscopes have inputs with selectable sensitivity settings, typically ranging from 1 mV/division To 5 V/division, using a combination of input attenuators and amplifiers in the signal path to an analogue to digital converter (ADC). A fast edge with a wide dynamic range is required to verify the performance of these selectable ranges. Of course, it would be possible to have a fixed amplitude for an, edge, and to effectively vary the amplitude to be applied to the equipment under test by manually attaching different combinations of coaxial attenuators to the output of the test signal generator. However, this is inconvenient in the context of automated calibration and verification of electronic equipment, for example when oscilloscopes are being manufactured or serviced.
An example of a commercially available advanced automated oscilloscope calibrator is that sold by Wavetek Corporation as model number 9500. This is a fully automated solution for oscilloscope calibration that provides many different waveforms and functions, including test signals with 150 ps edges, both rising and falling and with variable amplitudes ranging from 4.44 mV to 3.1 V. Such test signals are suitable for calibrating an oscilloscope having an input bandwidth of up to 2 GHz to 3 GHz. The edges produced by the model 9500 calibrator are ground-returning, whether rising or falling. This reduces the possibility of longer term changes in calibrator performance, since ground is a well-defined level.
A fast edge generator with wide dynamic range, such as say 60 dB, typically consists of a source whose dynamic range is about 10 dB followed by relay-switched attenuators, each attenuator having a resistor network. The performance that can be achieved with this design approach is limited not by the source itself, but by parasitic inductance and capacitance within the relays. These parasitics, which can be thought of as a departure from an ideal 50xcexa9 impedance transmission line, cause ringing and a loss of bandwidth in the test signal available from the output of the test signal generator. The best dual changeover relays currently available have an effective rise time of 50 ps each, equivalent to about 7 GHz, so that three attenuators in series would slow an infinitely fast edge to 87 ps. In addition, the materials used in the construction of these relays cause losses due to skin effect, resulting in a phenomenon called xe2x80x98dribble-upxe2x80x99. This gives a rounded corner after the edge, consistent with increasing loss at increasing frequency. This can be compensated for using networks of passive components, but tends to result in a loss of overall edge amplitude by as much as 25%. Furthermore, inexact compensation of such a large error adds to aberration uncertainties in the test signal edges.
It is an object of the present invention to provide an improved test signal generator.
Accordingly, the invention provides an electronic circuit for generating an electronic fast edge waveform of selectable amplitude, comprising: a waveform output from the circuitry; a plurality of attenuation stages, the attenuation stages being connected without any switching elements to each other to form a cascaded series of attenuation stages leading to the waveform output and each of the attenuation stages in the series being bounded by circuit nodes; or more pulse generators for producing an unattenuated source of the fast edge waveform; and means for selectively applying the fast edge waveform source to any selected one of the circuit nodes so that the fast edge waveform propagates from the selected circuit node through a selected number of attenuation stages to the waveform output thereby attenuating the fast edge.waveform to a selected amplitude.
In this specification, the term xe2x80x9cfast edgexe2x80x9d means an edge faster than about 350 ps, this being the rise or fall time equivalent to about 1 GHz.
An unattenuated fast edge waveform may be provided at the circuit output if one of the circuit nodes is after the last attenuation stage before the circuit output.
In a preferred embodiment of the invention, the attenuation stages are hardwired to each other. It would however, be possible to place certain types of passive component between the stages, for example a resistor or a coupling capacitor, as long as there are no relays or other types of switching elements such as diodes.
Because the attenuation stages are hardwired to each other in the sense that there are no relays or other switching elements between the attenuation stages, stray capacitance associated with such switching elements is avoided. As a result, degradation in the rise or fall times of the fast edge waveform is reduced.
In a preferred embodiment of the invention, there are a plurality of pulse generators, each pulse generator being individually activatable and connected to a different one of the circuit nodes, the means for selectively applying the fast edge waveform source to a corresponding selected one of the circuit nodes involving the selective activation of a selected pulse generator.
It would, however, be possible for there to be one or more pulse generators if there is a switching circuit between a pulse generator and a plurality of circuit nodes, the means for selectively applying the fast edge waveform source to a corresponding selected one of the circuit nodes involving the selective switching of the fast waveform source from said pulse generator via the switch to a selected one of the circuit nodes.
Although the switching element would introduce some additional stray capacitance, this would be less than in a conventional design in which relays have to be provided in series between successive attenuation stages. Such a switching element used with one pulse generator may also be more economical than a large number of pulse generators.
Preferably, pulse generators of opposite polarity are paired in connection to said circuit nodes. Thus both rising and falling fast edges can be produced from the same circuit.
In a preferred embodiment of the invention, each attenuation stage provides the same degree of attenuation to the fast edge waveform.
Each attenuation stage will provided discrete changes in waveform attenuation, for example with steps of 10 dB for every additional attenuation stage through which the fast edge waveform travels on its way to the circuit output. If a more continuous degree of attenuation is needed, then the pulse generator may be made so that it is operable to vary the amplitude of the fast edge waveform source. In a preferred embodiment of the invention, this is dote by varying the supply voltage to the pulse generator. Preferably, this ability to vary the amplitude of the fast edge waveform, is sufficient to span the discrete steps provided by the attenuation stages, so that any desired degree of attenuation can be selected between the attenuation steps.
The invention also provides a test signal generator for generating a fast edge electronic waveform, comprising electronic pulse generation circuitry according to the invention.